1. Field of the Invention
This invention relates to an expansion graphite sheet which is preferably usable for an exhaust gasket of engine, and also to the exhaust gasket.
2. Description of the Background Art
A conventional expansion graphite sheet would be fabricated as described in Patent Document 1, for example, where natural graphite is treated with sulfuric acid, nitric acid and phosphoric acid (which are interlayer intercalating agents and prepared in the form of solid or solution) to form graphite particles through the intercalation of the intercalating agents therein which are thermally treated later so as to be grown along the c-axis (vertically) and pressed to form the intended expansion graphite sheet. In the use of the expansion graphite sheet for an exhaust gasket, the expansion graphite sheet is cut out and molded in compression.
[Patent Document 1] W001/05703 international publication
Generally, the exhaust gasket would be used under a high temperature condition within a range of 600–700° C., and in contrast, the conventional expansion graphite sheet would be oxidized at about 500° C., so that if the exhaust gasket made of the conventional expansion graphite sheet is used under a high temperature condition of 600° C. or over, the exhaust gasket can not exhibit the sealing performance sufficiently due to the oxidization thereof.
In this point of view, it is desired to develop an expansion graphite sheet of high thermal resistance (high oxidation resistance) against a thermal condition within a range of 600–700° C.